The K. kagaensis fossil was unearthed in 2001, but it was only with recent analysis by Manabe and Susan Evan, a paleontologist at the University College London, that its significance was realized.

"We noticed the importance by looking at the teeth," Manabe said. "It's so rare, and it's so old."

The lizard's teeth were similar to those of modern day iguanas, which are one of the few fully herbivorous lizards alive today.

The researchers think the animal dined on some of the world's first flowers.

Modern gymnosperms such as ferns and gingko are typically not very nutritious and are tougher to chew than angiosperms.

"If you're a dinosaur, your jaw is bigger, so it's OK," Manabe said.

"But lizards are small, and they have delicate teeth, so it is very difficult for them to eat and chop off very tough material."

Alternatively, K. kagaensis might also have fed on young gymnosperm leaves, the researchers say.

"The big leaves are tough and fibrous," Manabe said. "But the young ones tend to be softer and smaller."

Christopher Austin, a herpetologist and curator at Louisiana State University's Museum of Natural Science, called the fossil discovery a "spectacular" find that challenges long-held views about lizard evolution.

"The ancestral condition for lizards has always been assumed to be insectivorous [insect-eating], so this new fossil provides data that challenges this thinking," Austin said.

He added that the anatomical features of K. kagaensis suggest "that either the ancestral condition for lizard diet was not as restricted as once thought or that diet has been highly labile [easily changed] throughout lizard evolution."